Nothing can happen, especially something like this, in Russia without Putin being involved. He's told the FSB to leave Wagner alone, much to their control freak annoyance. Wagner are linked to the military and GRU, who the FSB also dislike.

Basically, Wagner is called a private army but actually do Putin's bidding through the GRU but if any questions are asked Putin has plausible deniability kind of like saying "It's nothing to do with us. They're not part of the military." We know it's a lie it's so obvious, they've been seen mixing with ordinary Russian troops, but it's one of the games Putin plays. Using plausible deniability also how Putin gives orders in the Kremlin.

That is a double edged sword though. In a different conflict, when one Wagner group got too close to an American outpost the Americans contacted their Russian counterparts who disowned all knowledge of them. The Wagner group didn't turn back so the American troops opened fire with everything they had. They heard the radio chatter of the men as they realised the Russians had left them to die rather than to own up to being associated with them. They're very expendable.

The reason for using prisoners and mercenaries in this current war is because Putin believes the Russian public won't care if that type of people get killed rather than the general Russian public.

They're also being used as cannon fodder. Some prisoners are given a tiny bit of ammo, just enough to make a noise and attract attention, and then sent out. Their sole purpose is to get shot by the Ukrainians and thus give away their positions to the Russians who sent the prisoners out to be massacred. They don't give them much ammo as there's no point, they really are meant to get shot. If they try to escape they're publicly executed in a rather gruesome way.